1. Field of the Invention
This invention relates to the field of microstructure fabrication, and in particular to a method of aligning microstructures on opposite sides of a wafer.
2. Description of Related Art
The manufacturing of Micro-Electro-Mechanical Systems (MEMS) often requires the alignment of structures located on the front side of a silicon wafer to structures located on the back side of the same silicon wafer as to ensure proper machining.
Special alignment systems capable of back side alignment are used to perform the alignment of structures located on the front side of a silicon wafer to structures located on the back side of the same silicon wafer as to ensure proper machining of MEMS. These special systems use split field optics and/or through-the-silicon-wafer infrared imaging to perform 1xc3x97 direct contact alignment or 1xc3x97 proximity contact alignment of the structures on both sides of the silicon wafer.
One example of a prior art alignment system is the MA1006 contact aligner manufactured by KandW Gmbh. This system is capable of handling 1xc3x97 masks of sizes of up to 7xe2x80x3xc3x977xe2x80x3 and silicon wafers of up to 6xe2x80x3diameter. The transfer of a 1xc3x97 mask located on the back side of the silicon wafer into the photoresist also located on the back side of this silicon wafer while aligning this transferred 1xc3x97 photoresist pattern to the structures already located on the front side of the same silicon wafer is carried out by first illuminating the 1xc3x97 mask located on the back side of the silicon wafer with an intense infrared light not absorbed by the photoresist also located on the back side of the silicon wafer. Next, through infrared transmission, the dynamic alignment of the 1xc3x97 mask closely positioned against the photoresist coated back side of the silicon wafer and the structures located on the front side of the same silicon wafer is observed using the image transmitted through a 5xc3x97-35xc3x97 zooming microscope objective and captured from the Charge-Coupled-Device (CCD) infrared sensor of a Hamamatsu alignment camera a resulting infrared imaged magnified by 150xc3x97-1000xc3x97 zoom on a Sony 14xe2x80x3 Monitor. The 1xc3x97 mask and the photoresist coated back side of the silicon wafer are contacted with more or less pressure (0.03 N/cm2-0.16 N/cm2) when proper alignment is achieved. Finally, the photoresist coated on the back side of the silicon wafer is exposed with ultra-violet light of 240 nm-450 nm wavelength to transfer the pattern of the 1xc3x97 mask into the photoresist.
The resulting alignment of the already existing pattern located on the front side and of the newly transferred 1xc3x97 pattern into the photoresist of the back side of the wafer is claimed to be of the order of 1 xcexcm.
Another known system is the MA6 contact aligner manufactured by the company Karl Suss, also of Germany. This system is also capable of handling 1xc3x97 masks of sizes of up to 7xe2x80x3xc3x977xe2x80x3 and silicon wafers of up to 6xe2x80x3 diameter. In this system, the 1xc3x97 mask on the back side of the silicon wafer is first illuminated with infrared light. Using a splitfield CCD video camera and through infrared transmission, the alignment of the 1xc3x97 mask near the back side and the structures of the front side of the silicon wafer are observed using the image transmitted through a 10xc3x97 microscope objective. The 1xc3x97 mask and the photoresist coated back side of the silicon wafer are contacted with more or less pressure when proper alignment is achieved. Finally, the photoresist coated on the back side of the silicon wafer is exposed with ultra-violet light as to transfer the pattern of the 1xc3x97 mask into the photoresist.
The optical resolution of the transferred pattern into the photoresist is claimed to be of the order of 1 xcexcm when using vacuum contact between the 1xc3x97 mask and the photoresist.
Another system is the contact aligner also manufactured by Karl Suss. Unlike the MA6 aligner, which is capable of aligning wafers of up to 6xe2x80x3 in diameter, the MJB 3 UV400 IR 6 contact aligner is only capable of handling 1xc3x97 masks of sizes of up to 4xe2x80x3xc3x974xe2x80x3 and silicon wafers of up to 3xe2x80x3.
In this system, the 1xc3x97 mask on the back side of the silicon wafer is first illuminated with infrared light. Using a splitfield Vidicon video camera and through infrared transmission, the alignment of the 1xc3x97 mask near the back side and the structures of the front side of the silicon wafer is observed using the image transmitted through a 10xc3x97 microscope objective. The 1xc3x97 mask and the photoresist coated back side of the silicon wafer are contacted with more or less pressure when proper alignment is achieved. Finally, the photoresist coated on the back side of the silicon wafer is exposed with ultra-violet light as to transfer the pattern of the 1xc3x97 mask into the photoresist.
The optical resolution of the transferred pattern into the photoresist is also claimed to be of the order of 1 xcexcm when using vacuum contact between the 1xc3x97 mask and the photoresist.
The EV620 contact aligner manufactured by the company Electronic Visions is also capable of handling 1xc3x97 masks of sizes of up to 7xe2x80x3xc3x977xe2x80x3 and silicon wafers of up to 6xe2x80x3 diameter. In this system the 1xc3x97 mask on the back side of the silicon wafer is first illuminated. Using a splitfield video camera, the alignment of the 1xc3x97 mask near the back side and the structures of the front side of the silicon wafer is observed using the image transmitted through a 3.6xc3x97, 4xc3x97, 5xc3x97, 10xc3x97 or 20xc3x97 objective equipped with digital Zoom. The 1xc3x97 mask and the photoresist coated back side of the silicon wafer are contacted with more or less pressure (0.5 N to 40N) when proper alignment is achieved. Finally, the photoresist coated on the back side of the silicon wafer is exposed with ultra-violet light to transfer the pattern of the 1xc3x97 mask into the photoresist.
The alignment accuracy between the transferred patterns into the photoresist of the back side and the patterns of the front side is claimed to be better then about 1 xcexcm.
The OAI 5000 contact aligner manufactured by the company Optics Automation Instrumentation is capable of handling 1xc3x97 masks of sizes of up to 9xe2x80x3xc3x979xe2x80x3 and silicon wafers of up to 8xe2x80x3 diameter. In this system the 1xc3x97 mask on the back side of the silicon wafer is illuminated with infrared light. Using a splitfield video camera and through infrared transmission, the alignment of the 1xc3x97 mask near the back side and the structures of the front side of the silicon wafer is observed using the image transmitted through a 6xc3x97 or 32xc3x97 objective. The 1xc3x97 mask and the photoresist coated back side of the silicon wafer are contacted with more or less vacuum (2 to 15 inch of mercury) when proper alignment is achieved. Finally, the photoresist coated on the back side of the silicon wafer is exposed with ultra-violet light to transfer the pattern of the 1xc3x97 mask into the photoresist.
The optical resolution of the transferred pattern into the photoresist is claimed to be 0.73 xcexcm when using 365 nm i-line exposure of the 1xc3x97 mask into suitable photoresist.
All of these special alignment systems use split field optics and/or through-the-silicon-wafer infrared imaging to perform 1xc3x97 direct contact alignment or 1xc3x97 proximity contact alignment of the structures located on the front side of the silicon wafer and structures located on the back side of the same silicon wafer as to ensure proper machining of MEMS. They also all require physical contact between the photoresist and the 1xc3x97 mask using more or less pressure in order to align the patterns of the front side and the transferred 1xc3x97 pattern of the back side with alignment accuracy of the order of 1 xcexcm and optical resolution also of the order of 1 xcexcm.
The invention employs a special manufacturing technique which uses the crystal properties of the silicon wafer to permit the fabrication of thin transparent oxide islands on its front side over which opaque alignment structures are positioned in such a way to be visible through openings which are wet-etched through the surface of the back side and up to the bottom surface of this transparent oxide island. The alignment of the pattern-to-be-transferred on the back side of the facing down silicon wafer can be carried out using a standard 5xc3x97 Canon wafer stepper by aligning the alignment structures of the mask of the pattern-to-be-transferred to the alignment structures already located on the upper surface of the transparent oxide island.
Accordingly, the present invention provides a method of aligning structures on first and second opposite sides of a wafer, comprising the steps of forming at least one transparent island on the first side of said wafer at an alignment location, said transparent island having an exposed front side and a rear side embedded in said wafer; providing at least one alignment mark on the exposed front side of said transparent island; performing an anisotropic etch through the second side of said wafer to form an opening substantially reaching the rear side of said transparent island; and carrying out a precise alignment on said alignment mark through said opening and said transparent island.
There can be one large island with several alignment marks, but more usually there several smaller islands will be formed, each bearing one or more alignment marks. These are typically opaque structures, for example, of polysilicon.
The openings on the second side of the wafer should normally reach the transparent islands, although the invention will still work so long as they extend sufficiently far to permit the alignment structures to be visible through the openings from the second side of the wafer.
The transparent islands are preferably made using a LOCal Oxydation of Silicon (LOCOS) sequence in a MEMS process as to allow the fabrication of oxide islands on the front side of the water. These are then used as an etch stop layer during etching of the openings, which form observation windows, from the back side of the wafer at a later step of the process.
The alignment structures can be patterned over these oxide islands using a standard Canon 5xc3x97 wafer stepper so as to be visible from the back side of the wafer through the underlying oxide islands and through the observation windows which cab be etched from the back side of the wafer at a later step of the process.
Preferably, a chemical solution is used to transfer a first poorly aligned thermal oxide hard mask using the major flat and the edges of the silicon wafer as the positioning reference. This hard mask is used to open coarsely aligned observation windows with this chemical solution which performs an anisotropic etching along some crystallographic planes from the back side and all the way through the 625 xcexcm thick silicon wafer as to reach the oxide islands. These observation windows allow the observation of the alignment structures of the front side of the wafer from the back side of the wafer.
The alignment structures of the mask to be aligned on the back side of the wafer can then be properly aligned through these coarsely aligned observation windows with their corresponding alignment structures located over the grown oxide island on the front side of the wafer using a standard 5xc3x97 Canon wafer stepper. The transferred patterns on the back side of the wafer can also include other alignment marks which can allow shot-by-shot alignment of the remaining masks to be transferred to the back side of the wafer.